Image intensifiers



IMAGE- INTENSIFIERS Filed May 14, 1968 s Sheets-Sheet 1 INVENTOR BRIANWILLIAM MANLEY ZQ-AZ I I Fab. 24,1970 I I BQW MANLEY 7 $497,759

' IMAGE INTENSIFIERS I mea'ua 14, 1968 '3 Sheets-Sheet 2.

INVENTOR BRIAN WILLIAM MNLEY Filed May 14, 1968 3 Sheets-Sheet 3 UnitedStates Patent 3,497,759 IlVIAGE INTENSIFIERS Brian William Manley,Burgess Hills, England, assignor, by mesne assignments, to US. PhilipsCorporation, New York, N.Y., a corporation of Delaware Filed May 14,1963, Ser. No. 729,126

Claims priority, application Great Britain, May 15, 1967,

22,339/67 Int. Cl. HOlj 31/48 US. Cl. 315-11 7 Claims ABSTRACT OF THEDISCLOSURE This invention relates to electron multiplier and imageintensifier devices. More particularly the invention relates to channelintensifier devices and to electronic tubes employing such devices.

A channel intensifier device is a secondary-emissive electron multiplierdevice which device comprises a resistive matrix in the form of a platethe major surfaces of which constitute the input and output faces of thematrix, a conductive layer on the input face of the matrix serving as aninput electrode, a separate conductive layer on the output face of thematrix serving as an output electrode, and elongated channels eachproviding a passageway from one face of the assembly consisting ofmatrix and input and output electrodes to the other face of saidassembly.

In the operation of such intensifier devices a potential difference isapplied between the two electrode layers of the matrix so as to set upan electric field to accelerate the electrons, which field establishes apotential gradient created by current flowing through resistive surfacesformed inside the channels or (if such channel surfaces are absent)through the bulk material of the matrix. Secondary-emissivemultiplication takes place in the channels.

With such devices the distribution and cross-sections of the channelsand the resistivity of the matrix are such that the resolution andelectron multiplication characteristics of any one unit area of thedevice is sufiiciently similar to that of any other unit area for anyimaging purposes envisaged.

If such a device is used in an imaging tube or system, the latter willbe referred to for convenience as an image intensifier tube or systemrather than as an image converter tube or system even in applicationswhere the primary purpose is a change in the wavelength of the radiationof the image.

British patent specifications 1,064,073, 1,064,074 and 1,064,076describe examples of a channel intensifier device used in conjunctionwith a photo-cathode spaced from the input electrode and with a suitabletarget, for example a luminescent screen so as to form an arrangementsuitable for an image intensifier tube, for example for viewing scenesat low illumination.

In further arrangements described in French patent specification1,404,980 the photo-cathode is no longer 3,497,759 Patented Feb. 24,1970 spaced from the channel intensifier device. Such arrangementsemploy the channel intensifier device in combination with photo-emissivesurface areas in contact with the input electrode of the device.

The photo-emissive surface areas may substantially all be formed on theinput electrode of the matrix and they may constitute an electricallycontinuous apertured layer, which can be represented as the layer P inFIG- URE 1 of the accompanying diagrammatic drawings. An object O isshown imaged by an optical system on to the photo-cathode P.Photo-electrons are liberated simultaneously from all parts of thephoto-cathode with varying local intensities dependent upon the imageformed thereon. Secondary electrons emerging from channel intensifierdevice I are accelerated towards a luminescent screen S.

More particularly, the channel intensifier device I is traversed by aregular array of channels. The matrix of the device may be of glass andits input and output faces carry first and second conductive electrodelayers E1-E2 respectively.

In each of the channels that receives primary electrons at any giveninstant, multiplication takes place and the necessary electricaccelerating field is set up by connecting the electrodes El-EZ to asource shown schematically at B2. A further accelerating field isprovided by a source shown schematically as a unit B3 connected betweenE2 and a conductive coating (e.g. aluminum) associated with luminescentscreen S.

Photo-electrons are emitted in a direction away from the matrix andinput electrode E1 and such electrons require a field to turn them backtowards the channels. Means for producing such a field are representeddiagrammatically by a source B1 applying a voltage between the inputelectrode E1 and a transparent electrode E0 formed e.g. on the envelope.In practice it is found that the field configuration existing at theentrances to the channels due to the elements E1-E2B2 alone can besufficient to draw back the photo-electrons without the need for theelectrode E0 and source B1.

As an alternative to location on the input electrode, the photo-emissivesurface areas is shown to be formed substantially entirely within thechannels of the matrix. In this case the accelerating field set up bysource B2 between electrodes El and E2 is clearly suflicient toaccelerate the electrons in the channels without the need to have anelectrode corresponding to E0 with its source B1.

As a further alternative the photo-emissive surface areas have been laidpartly on the input electrode on the matrix and partly inside itschannels.

There is a problem associated with imaging tube constructions whichoperate with an electron accelerating field to direct electrons frompoints on the photo-cathode to corresponding points on the inputelectrode. This problem is that the accelerating field tends to directthe electrons into the channels at high speed in a direction more orless parallel to the axes of the channels. Consequently there is atendency for the electrons to fail to strike the channel walls at anyearly stage so that less multiplication steps occur and the totalmultiplication effect is reduced. A second problem exists in thatelectrons emitted from any given picture element on the photo-cathodeare apt to spread and enter more than one channel.

According to the invention with a channel image intensifying device forelectrons comprising a thin plate of glass of high electrical resistanceor of a ditfe r ent kind of similar material, which plate is provided onthe two major surfaces with an electrically conductive layer and withclosely adjacent channels interconnecting the two surfaces and with aphoto-electric cathode in contact with one of the two surfaces, thephoto-electric cathode closes the passages at the entrances to thechannels and is sufliciently permeable for rays of those wavelengths towhich the photo-cathode material is sensitive.

The entrances to all the channels are closed by photoemissive areas, andthe device can operate satisfactorily even if some parts of the inputelectrode are not quite in direct physical contact with thephoto-emissive layer owing, say, to irregularities in said layer or insaid electrode or to the presence of dust particles. This is trueprovided that such photo-emissive layer is in electrical contact withthe input electrode even though they are not in physical contact, andthis can readily be achieved by forming the photo-emissive layer as anelectrically continuous layer having sufficient conductivity to maintainall the areas effectively at the same potential as the input electrode.In practice it is possible to carry out such a construction with suchaccuracy that areas of imperfect physical contact between inputelectrode and photo-emitter cause only negligible local losses ofresolution due to a few electrons entering the wrong channels.

The invention also overcomes the aforesaid second problem which existsin previous arrangement of the proximity type in that electrons fromeach picture element on the photo-cathode are constrained to enter theappropriate channel, and this of course produces maximum definition fora given channel density or for a given total number of channels.

Embodiments of the invention will now be described by way of examplewith reference to FIGURES l to 10 of the accompanying diagrammaticdrawings in which FIGURE 1 illustrates the prior art,

FIGURE 2 represents schematically an image intensifier tube according tothe invention,

FIGURES 3 to 5 illustrate 3 embodiments of the invention,

FIGURE 6 illustrates a modification of the invention,

FIGURE 7 illustrates a method of manufacture,

FIGURES 8 and 9 illustrate in a simplified manner the action ofsymmetrical and asymmetrical (i.e. tilted) lenses respectively,

FIGURE 10 illustrates a further embodiment.

In the generic representation of an imaging tube employing achannel-intensifier photo-cathode combination according to the presentinvention as shown in FIGURE 2 the input voltage supply B1 and theseparate electrode E0 of FIG. 1 have been omitted. Though thephotoemissive material has a degree of conductivity of its own theoriginal input electrode E1 is retained as part of the channelintensifier device. Electrode E1 communicates its own potential to thephotoemissive areas since they are in contact with it as explainedabove. Thus the photoemitter and input electrode can be indicated asbeing connected together to one end of the supply B2.

As shown in an enlarged schematic manner in FIGURE 3 a continuousphoto-emissive layer P is brought up to, and placed in contact with, theinput electrode E1. The parts of the photo-emissive layer whichcorrespond to the electrode E1 perform no photo-emissive function in thedevice. In some cases it may be the easiest and cheapest method ofconstruction by depositing layer P on a glass plate W which may be thewindow of the envelope since the photo-emissive surface areas can beproduced as a continuous layer before assembly. The areas of thephoto-emitter which correspond to the channels C are the operative areasand they emit photo-electrons directly into the channels. This permitsthe electrons to initiate their travel in the channels at a lower energyso that they can more easily be directed towards the channel walls at anearly stage. This can be done in different ways and these alternativearrangements are illustrated in FIG- URES 4- and 5. In FIGURE 4 theinput electrode E1 is extended some way into each channel entrance so asto create an electrostatic lens effect which deflects the electronsoutwardly towards the channel Walls. This lens action will be describedin greater detail later.

In the alternative arrangement of FIGURE 5 the channels are tilted so asto cause early collisions as shown schematically. It is possible tocombine the configurations of FIGURES 4 and 5 (although this is notnormally necessary) or the lenses themselves may be tilted as will beexplained.

In any of the arrangements of FIGURES 3 to 5 it is possible in principleto eliminate those parts of the photoemissive layer P which correspondto the input electrode E1 and are therefore not utilised forphoto-emission. However, this is extremely difficult to carry out withpresent techniques and requires sufiicient precision of manufacture forsubstantially all the photo-emissive areas to be individually inphysical contact with the input electrode since otherwise chargedeposition and like phenomena will disturb the operation of the device.

A modification of the present invention consists in omitting the inputelectrode E1 and relying solely on the conductivity of thephoto-emissive areas to act as an input electrode. For this reason areasmust, as in the arrangements of FIGS. 35, be joined together to form anelectrically continuous layer which is connected to one of the supplyterminals. An example of such an arrangement is shown in FIGURE 6.

It is in accordance with the definition given above of a channelintensifier device that in the arrangement of FIGURE 6 the inputelectrode is regarded as constituted by the photo-emissive layer whereinthe input electrode is no longer traversed by the passageways providedby the channels.

A special case of the FIGURE 6 type is the case in which the concernedphoto-emissive and electrode material PE is a metal adapted to act as aphoto-emitter at given wavelengths of input radiation, for example goldfor an ultra-violet image intensifier. However, such a metallic layermust be thin enough to be partially transparent and this may undulylimit its current carrying capacity as compared with the arrangements ofFIGURES 3 to 5 wherein the electrode E1 has apertures to pass radiationand therefore can be of any desired thickness. A similar limitation mayexist also when the layer PE of the arrangement of FIGURE 6 is of amaterial other than gold.

In manufacturing the devices described, the electrode E1 can be formedon the matrix by known methods. If it is to be extended into thechannels in accordance with FIGURE 4, it can be formed by evaporating ametal (e.g. chromium) at a suitable angle 5 as shown in FIG- URE 7, suchevaporation (3) being effected from a source which is rotated round theaxis of the matrix so as to cause uniform penetration the channel plateis itself rotated (or relative to a fixed source). The chosen value ofthe angle 5 of evaporation determines the depth of the inwardpenetration d of the electrode material inside the channels 2. Thisforms both the input face layer 1 of the electrode and the extensions 4of the electrode into the channels.

Arrangements such as those of FIGURES 3 to 5 may be made by depositingthe layer P on to a substrate plate W and assembling said plate againstthe electrode E1 of the channel device. This must be done withsufiicient accuracy to ensure that layer P is in contact with all ornearly all the parts of the electrode, and the two steps should becarried out in vacuo. Corresponding steps can be adopted for the case ofFIGURE 6 but in this case the assembly can be carried out in air if thelayer PE is of gold as previously described.

The lens effect previously referred to in connection with the electrodeextensions of FIGURE 4 will now be de scribed in greater detail withreference to FIGURE 8 after a brief review of the problem associatedwith the channel entrance conditions in prior channel imageintensifiers.

Electrons produced from the photocathode (in response to light or otherradiation) must acquire so much energy that, when they strike the wallof a channel, the

secondary emission coefficient will be substantially greater than unity.This requires in practice collision energies which exceed 50 ev. It isalso important, however, that electrons from the photocathode do notpenetrate far into the channel before collision with the wall, since thelength of channel available for the subsequent gain process will beinadequate. Usually a channel plate is separated from the photocathodeby a small distance (between one and ten channel diameters), and theapplication of a potential difife rence exceeding 50 v. (betweenphotocathode and channel plate input electrode) to ensure thatphotoelectrons enter a channel with sufficient energy to producesecondary electrons on collision with the wall of the channel. To causecollision with the wall in the early part of the channel the fieldstrength in the channel plate is made different from that in the spacebetween photocathode and channel plate. Thus a lens action isestablished at the channel entrance, which will be positive if the fieldstrength in the channel plate is the higher, or negative if it is thelower. Unfortunately, if the strength of this lens is to be adequate toensure that electrons are directed to strike the wall in the early partof the channel, it is necessary that either the disparity in fieldstrengths be very great, or the energy of the electron as it enters thelens be small, so that it is readily deflected. The electric fieldwithin the channel plate is normally fixed by the gain required from theplate and its geometry, and there is little or no freedom to choose thefield strength to suit electron-optical requirements. Thus to achievedisparity in field strengths, either the field strength in thephotocathode/channel-plate ga must be made very high (in which casefield emission from the photocathode becomes a danger) or it must bemade very low, in which case the electrons from the photocathode willspread before reaching the channel plate and resolution will be lost.

In arrangements according to the present invention, wherein the channelplate is in contact with the photomissive areas and said areas close thechannel entrances, this ensures that the resolution is limitedsubstantially only 'by the channel spacing rather than by electronspreading from the photocathode.

By arranging that the input electrode of the channel plate penetrates asmall distance into the channel as shown in FIGURE 4, a virtuallyfield-free region is established in the region of the photocathode. Thisphotoelectrons enter a converging electron lens occurring at theboundary of the penetrating electrode extension. These electrons havesmall energy and the lens can thus be adequately strong to ensure earlywall collision for most photoelectrons.

The degree of penetration of the electrode will influence the lensstrength. As a limit case, no penetration will result in no lens action,and the photoelectrons will spread in straight paths from thephotocathode. Deep penetration several channel diameters in extent willgive strong curvature of the equipotentials and a strong lens action;however, many photoelectrons will drift to the wall in the electroderegion without gaining any energy and so will be lost. The bestcompromise appears to lie with a penetration between A and 2 channeldiameters in depth, preferably between /2 and one diameter. In thisrange a large fraction of the photoelectrons will gain sufficient energy(before collision) to produce secondary electrons, and few still passfar into the channel without collision.

The lens action is shown diagrammatically in FIG- URE 8. In order toreduce the fraction of axial electrons which still proceed aconsiderable distance into the channel before colliding, the electrodepenetration forming the lens can be tilted according to FIGURE 9. Thishas the result of accelerating the electrodes preferentially to one sideof the channel, and even a photoelectron emitted along the channel axiswill collide with the wall.

Tilted electrode penetration (i.e. tilted lenses) can be produced byevaporation of metal forming the input electrode (typically Cr) at asuitable angle. Referring back to FIGURE 7 it was explained that, byrotation of the plate during exaporation, substantially uniformpenetration can be achieved. For the present purpose, skew penetrationlike that shown in FIGURE 9 can be obtained by omitting the rotation.

For a given total matrix area and given channel diameters density, theeffective photocathode area of any of the devices described can beincreased by outwardly flaring or tapering the entrances to the channelsso that their initial diameter is larger. This is illustratedschematically in FIG. 10.

In a practical example the dimensions of the matrix may be approximatelyas follows:

Diameter of matrix.3l0 cm. Diameter of channel-15 Length of a channel.1mm.

For use in an image intensifier the source B may produce about 1000volts.

What is claimed is:

1. An electron multiplier comprising a thin plate of electricallyinsulating material, said plate having an electrically conducting layeron the two major faces and being provided wi.h a plurality of parallelclosely adjacent narrow secondary emissive channels interconnecting thetwo major faces, and a photoelectric cathode contacting one of the twomajor faces, and closing the entrances to the channels and sufiicientlypermeable to radiation of such wavelengths to which the photocathodematerial is sensitive to emit electrons in response to said radiationwhich enter said channels.

2. A device as claimed in claim 1, wherein the photocathode is formed bya continuous layer applied to a transparent support.

3. A device as claimed in claim 2, characterized in that between thephotocathode and the plate surface a conductive layer is provided whichextends into the channels.

4. A device as claimed in claim 3, wherein the length of the extensionof the conductive layer in the channels is equal to /2 times a channeldiameter.

5. A device as claimed in claim 3, wherein the layer extension in achannel terminates in the form of a bevelled cylinder.

6. A device as claimed in claim 1 wherein with respect to the majorfaces the channels extend in an oblique direction.

7. A device as claimed in claim 1, the photocathode is also theelectrically conducting layer.

References Cited UNITED STATES PATENTS 2,942,133 6/1960 McGe'e 3151l X3,001,098 9/1961 Schneeberger 3l511 3,374,380 3/1968 Goodrich 313X3,407,324 10/1968 Rome 313105 X RODNEY D. BENNETT, JR., Primary ExaminerJEFFREY P. MORRIS, Assistant Examiner US. Cl. X.R. 313-105

